drm/i915: Disable framebuffer compression for i915 driver in VM

Framebuffer compression is disabled when driver detects it's
running in a Intel GVT-g enlightened VM, because FBC is not
emulated and there is no stolen memory for a vGPU.

v2:
take Chris' comments:
        - move the code into intel_update_fbc()

v4:
take Tvrtko's comments:
        - rebase the code into intel_fbc_update()

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhiyuan Lv <zhiyuan.lv@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_fbc.c

@@ -505,6 +505,10 @@ void intel_fbc_update(struct drm_device *dev)
 	if (!HAS_FBC(dev))
 		return;
 
+	/* disable framebuffer compression in vGPU */
+	if (intel_vgpu_active(dev))
+		i915.enable_fbc = 0;
+
 	if (i915.enable_fbc < 0) {
 		if (set_no_fbc_reason(dev_priv, FBC_CHIP_DEFAULT))
 			DRM_DEBUG_KMS("disabled per chip default\n");